1. Technical Field
The present invention relates to a method of manufacturing a porous pre-sintered granule for a porous sintered reaction-bonded silicon nitride and a porous pre-sintered granule manufactured by the method. More particularly, the present invention relates to a method of manufacturing a porous pre-sintered granule, to which a pressure forming technology can be applied to obtain a porous sintered reaction-bonded silicon nitride having high porosity and having a structure in which macropores and micropores coexist with each other, and to a porous pre-sintered granule manufactured by the method.
2. Description of the Related Art
Porous silicon nitride materials have been widely used in the fields requiring excellent thermal, mechanical and chemical-resistant characteristics because they are light and have excellent strength, toughness, impact resistance, heat resistance, and corrosion resistance.
Conventionally, porous silicon carbide materials are generally used in the fields requiring excellent thermal, mechanical and chemical-resistant characteristics. However, these porous silicon carbide materials are problematic in that, since they have relatively low strength and impact resistance and high hardness, a mold is worn during a forming process, thus shortening the life span of the mold, and in that they are sintered at a high temperature of 2000° C. or more, thus causing an increase in production cost.
As described above, porous silicon nitride materials are promising materials that can be effectively used in filters for separating fine dust from diesel vehicles, catalyst carriers, heat insulation materials and the like, because they have excellent heat resistance, mechanical properties and corrosion resistance.
Conventionally, most research into silicon nitride materials has been focused on improving the mechanical and thermal properties thereof by densifying the microstructures thereof. Therefore, research into methods of manufacturing a porous silicon nitride material is relatively insufficient.
As examples of methods of manufacturing a porous silicon nitride material, Korean Unexamined Patent Application Publication No. 1995-702510 provide a method of manufacturing a porous silicon nitride ceramic material comprising a compound of silicon nitride (Si3N4) and a rare-earth element and/or a transition metal compound, wherein the porous silicon nitride ceramic material is used as a filter for removing foreign matter or a catalyst carrier. In this method, a molded powder mixture is heat-treated at a temperature of 1500° C. or more to obtain a porous silicon nitride ceramic material having a porosity of 30% or more.
Further, Korean Patent Registration No. 10-0311694 provides a method of manufacturing a sintered porous silicon oxynitride material used in a refractory tile or the like for a space shuttle. The method comprises the steps of: agglomerating a low-melting powder mixture containing Si3N4 11˜16 wt %, AlN 3˜5 wt %, Al2O335˜45 wt % and Y2O3 35˜45 wt %; adding the agglomerated low-melting powder mixture to β-sialon silicon oxynitride powder containing Si3N4 57˜100 wt %, AlN 0˜33 wt %, Al2O3 0˜9 wt %; and sintering the mixture at 1600˜1700° C. for 1˜8 hours.
Meanwhile, Japanese Unexamined Patent Application Publication No. H09-100179 discloses a method of manufacturing a porous silicon nitride material that can be used as a filter or a catalyst carrier. In this method, the porous silicon nitride material is manufactured by bringing a porous material containing silicon nitride as a main component into contact with acid and/or alkali to partially or entirely dissolve the components other than silicon nitride.
However, the above-mentioned methods are problematic in that they cannot be easily put to practical use because expensive silicon nitride is used as a raw material, and in that processes for forming pores are unpractical. For example, in the Korean Patent Registration No. 10-0311694, pores are formed by agglomerating low-melting powder and then mixing the agglomerated low-melting powder with high-melting powder. That is, the formation of pores depends on the size of the agglomerated low-melting powder. Therefore, this method is problematic in that it is difficult to maintain the form of the agglomerated low-melting powder when the agglomerated low-melting powder is mixed with the high-melting powder, and it is difficult to sufficiently mix them with each other when the form of the agglomerated low-melting powder is to be maintained constant, and in that it difficult to consistently control processes, and costs increase. Further, in the Japanese Unexamined Patent Application Publication No. H09-100179, pores are formed by chemical treatment. However, this method is also problematic in that chemical treatment, as an additional process, must be conducted, and in that it is not easy to maintain the structure of silicon nitride when the components other than silicon nitride are dissolved.
Meanwhile, Korean Unexamined Patent Application No. 2009-107392 provides a method of manufacturing a porous pre-sintered granule by granulating a raw material containing silicon and sintering additives for preparing silicon nitride and then pre-sintering the granule of the raw material. The porous pre-sintered granule manufactured by this method has the necessary strength to keep the spherical morphology during the shaping process. Therefore, the porous pre-sintered granule can be used to manufacture porous sintered silicon nitride material having excellent air permeability and trapping efficiency. However, this method is also problematic in that optimal process conditions must be established to allow a porous pre-sintered granule to have sufficient strength during press forming so that the designed pore structure and porosity are maintained even after sintering.